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Science and Technology K–6 · Year 4

Water cycle in a bag on a sunny window

Science understanding: Earth and space sciences

PracticalLow risk

The idea

Water evaporates when warmed, condenses on a cooler surface and falls back as drops, the same evaporation, condensation and precipitation that move water through the sky, land and ocean.

What you need

  • 1 large zip bag (27 x 27 cm)
  • 60 mL water with 2 drops of blue food colouring
  • permanent marker
  • tape
  • a sunny window and a thermometer taped beside the bag

How to do it

  1. Draw a sun, a cloud and an ocean on the bag with the marker.
  2. Pour in the coloured water, squeeze out most of the air and seal the bag.
  3. Tape the bag to a window in full sun with the water at the bottom. Record the time and the temperature at the window.
  4. Look every 30 minutes for 3 hours and record what you see on the inside of the bag: mist, drops, drops running down.
  5. Measure the temperature each time. Note where the drops form (the cooler top and sides) and where they run to.
  6. Move the bag to a shaded window for a day and repeat. Compare how long the drops took to appear.

What you should see

During the 3 hours in sun the upper inside of the bag fogs, then clear drops form and run down the sides into the ocean. The drops are clear, not blue: only water evaporates and the dye stays behind. In shade the drops take far longer or do not appear, because the warmer water in sun evaporates faster and the glass side is cooler than the water, so vapour condenses there.

What changes

What you change
sun or shade
What you measure
time until drops appear
What you keep the same
  • water volume
  • bag
  • window

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

  • The drops are the blue water leaking through the bag (they are clear, distilled by evaporation).
  • Clouds are made of water vapour (they are liquid drops that have condensed).
  • Rain comes from the sea only.

Safety card

Low riskLearners carry it out

Hazards

  • food colouring stains

Controls

  • seal the bag well
  • tape over a sink or tray if possible

Note

Risk assessment before the lesson using Primary RiskAssess or the school's own template.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

  • Science and Technology K-6 Syllabus (2017), current, taught until 2026; the 2017 syllabus has no water-cycle content; evaporation and condensation fall under this Stage 2 changes-of-state outcome ('recognise that a change of state can be caused by adding or removing heat'); code read from the syllabus document on 22 September 2026ST2-6MW-S
  • Science and Technology K-6 Syllabus (2017), current, taught until 2026; code read from the syllabus document on 22 September 2026ST2-1WS-S
  • Science and Technology K-6 Syllabus (2024), implemented from 2027; NESA's timeline is 2026 plan and prepare and 2027 start teaching, and schools may choose to implement it during 2026; code read from the outcomes page on 22 September 2026ST2-PQU-01
  • Australian Curriculum v9AC9S4U02AC9S4I01AC9S4I03

Sources

The pages the author read to write this activity.

  1. www.scootle.edu.au/ec/search?accContentId=AC9S4U02
  2. curriculum.nsw.edu.au/learning-areas/science/science-and-technology-k-6-2024/outcomes
  3. www.nsw.gov.au/education-and-training/nesa/curriculum/science/science-and-technology-k-6-2017
  4. primaryconnections.org.au/v84-sequences/water-works
  5. edu.rsc.org/primary-science/how-to-purify-water/4012058.article

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